Mulches Used in Highbush Blueberry and Entomopathogenic Nematodes Affect Mortality Rates of Third-Instar Popillia japonica

SIMPLE SUMMARY: Japanese beetle is a serious pest of many crops, including blueberry. While the adult beetles feed on leaves, the grubs (immatures) feed on plant roots and can cause wilting and reduced plant growth. Farmers rely on insecticides to control Japanese beetle, but nematodes that attack and kill grubs in the soil may be as effective as insecticides in some cases. Mulches of compost, woodchips, sawdust, and tree bark are commonly used in blueberry production, but it is not known how mulches affect Japanese beetle grubs or the effectiveness of beneficial nematodes to control grubs. We placed grubs in soil beneath different mulches in the laboratory and tested two nematodes. The species H. bacteriophora killed almost all grubs in all mulches, but S. scarabaei was not as effective and was more affected by mulch type. A mulch of compost + woodchips and sawdust caused 60% grub mortality without adding nematodes. In a field experiment during October, the nematodes caused 50% grub mortality, which was lower than expected and likely due to cool soil temperatures. We recommend using H. bacteriophora for Japanese beetle grub management in blueberry, regardless of the mulch type being used. ABSTRACT: Popillia japonica Newman (Japanese beetle) is an invasive, polyphagous pest in North America, as adults feed on plant foliage and larvae on roots. Management in crops relies on foliar and soil applications of insecticides, but entomopathogenic nematodes (EPN) are effective biocontrol agents. In highbush blueberry, mulches (composts, woodshavings, sawdust, bark) are used for weed control and fertility. Therefore, our objective was to determine the effects of Heterorhabditis bacteriophora and Steinernema scarabaei on third-instar P. japonica in substrates commonly used as mulches in blueberry. In containers in the laboratory, larval mortality was 90–100% with H. bacteriophora for all substrates, but rates with S. scarabaei were lower and variable among substrates. A mixture of municipal compost + woodchips/sawdust resulted in 60% larval mortality without adding EPN, but few nematodes were recovered, indicating other causes of death. In a field microplot experiment in October, larval mortality rates were 50% at most for all EPN and substrate type combinations, likely due to lower than optimal soil and substrate temperatures for EPN survival and infectivity. Overall, a compost and woodchip/sawdust mulch should help suppress P. japonica populations in blueberry, and applying H. bacteriophora when temperatures are optimal to mulches can provide excellent larval control.